Diodes are rectifying elements which conduct a current when they are forward biased and which block when they are reverse biased. When, however, the reverse voltage is higher than a breakdown voltage of the diode, a current can also flow in the reverse direction. Some types of diodes, like Zener diodes or avalanche diodes have a well defined breakdown voltage, which makes these diodes suitable to be used as voltage limiting elements or as reference voltage generating elements.
The breakdown voltage in conventional diodes is mainly given by the doping concentrations of the individual semiconductor regions that form the diode and by the diode layout. Thus, the breakdown voltage in conventional diodes is fixed. However, especially in applications in which a diode is used as a voltage limiting element or as a reference voltage generating element, it may be desirable to vary the breakdown voltage within a given range.